To reproduce:
Open new document
In Styles and Formatting pane, Organizer tab, amend Heading 1 to have 'next
style' Heading 2
Do Tools > Outline Numbering and set Heading 1 and Heading 2 numbering to any
choice except none, leaving 'Show sub-levels' at the default (2 at level 2, 3 at
level 3...).
Insert two paragraphs with any text. Apply Heading 1 style to the first and
Heading 2 style to the second. The paragraphs are automatically numbered correctly.
Put cursor at the end of the Heading 1 paragraph, immediately before the
paragraph mark.
Press return and note corrupted heading levels (eg Heading 1 paragraph numbering
now has 2 sub-levels instead of 1).
Do Tools>Outline numbering>OK to correct the display.

Similar results for any Heading level with 'next style' set to any other
Heading. For example, after pressing Return at the end of a paragraph with
Heading [n] style applied, Writer displays outline numbering with the number of
sub-levels in the Heading set as 'next style' in Heading [n]. 
Tools>Outline numbering>OK always puts things right.

This strange display behaviour is a reason for a legal firm not to adopt OOo
because it is irritating and confusing for their administrative staff when
amending contract documents.